A device for the connection of external lines to a programmable logic controller is described in German Patent No. 43 23 440 A1, the device being coupled to the programmable logic controller for signal transmission and being capable of being installed separately from it. The terminal unit allows for a modular structure in which a terminal is available for each actuator/sensor connection, it being correspondingly possible to provide each connection type with an electronic module for signal conversion. Input and output modules can thus be mixed as desired. This has an advantageous effect on project planning, installation, troubleshooting or expansion of the electronic device. At the same time, the terminal unit functions as a support unit for the electronic modules and has a plurality of identical plug receptacles for them. Both feeder and signal lines are connected to the electronic modules via a plug-in connector at the plug receptacle. Electronic modules with different operating voltages, e.g., 230 V alternating current or 24 V direct current, can be used. The wiring via the plug-in connector at the plug receptacle does indeed facilitate the replacement of electronic modules; however, at the same time. It also increases the risk of confusing them. This can result in problems, since the particular type of the electronic module to be inserted in a plug receptacle, its supply voltage in particular, is determined after wiring of the device for the connection of external lines has been completed.